Cellulosic biomass is a significant renewable resource for the generation of sugars. Fermentation of these sugars can yield commercially valuable end-products, including biofuels and chemicals that are currently derived from petroleum. While the fermentation of simple sugars to ethanol is relatively straightforward, the efficient conversion of cellulosic biomass to fermentable sugars such as glucose is challenging. See, e.g., Ladisch et al., 1983, Enzyme Microb. Technol. 5:82. Cellulosic biomass may be pretreated chemically, mechanically or in other ways to increase the susceptibility of cellulose to hydrolysis. Such pretreatment may be followed by the enzymatic conversion of cellulose to glucose, cellobiose, cello-oligosaccharides and the like, using enzymes that specialize in breaking down the β-1-4 glycosidic bonds of cellulose. These enzymes are collectively referred to as “cellulases”.
Cellulases are divided into three sub-categories of enzymes: 1,4-β-D-glucan glucanohydrolase (“endoglucanase” or “EG”); 1,4-β-D-glucan cellobiohydrolase (“exoglucanase”, “cellobiohydrolase”, or “CBH”); and 3-D-glucoside-glucohydrolase (“β-glucosidase”, “cellobiase” or “BG”). Endoglucanases randomly attack the interior parts and mainly the amorphous regions of cellulose. Exoglucanases incrementally shorten the glucan molecules by binding to the glucan ends and releasing mainly cellobiose units from the ends of the cellulose polymer. β-glucosidases split the cellobiose, a water-soluble β-1,4-linked dimer of glucose, into two units of glucose. In order to efficiently convert cellulosic biomass to fermentable sugars, a complete cellulase system comprising components from each class of cellulases (CBH, EG and BG) is required, and it is well known in the art that the individual cellulase components are less effective at hydrolyzing cellulose comprising substrates than the combination of the three components.
Although cellulase compositions have been previously described, there remains a need for new and improved cellulase compositions for use in processing cellulosic biomass and for cellulase compositions that contribute to the reduction in cost and increase in efficiency of producing biofuels and other commercially valuable compounds.